Try out CircleCI. (#630)

.circleci/config.yml

@@ -0,0 +1,38 @@
+workflows:
+  version: 2
+  main:
+    jobs:
+      - go-current
+      - go-previous
+      - go-latest
+base: &base
+  working_directory: /go/src/github.com/spf13/cobra
+  steps:
+    - checkout
+    - run:
+        name: "All Commands"
+        command: |
+          mkdir -p bin
+          curl -Lso bin/shellcheck https://github.com/caarlos0/shellcheck-docker/releases/download/v0.4.3/shellcheck
+          chmod +x bin/shellcheck
+          go get -t -v ./...
+          PATH=$PATH:$PWD/bin go test -v ./...
+          go build
+          diff -u <(echo -n) <(gofmt -d -s .)
+          if [ -z $NOVET ]; then
+            diff -u <(echo -n) <(go tool vet . 2>&1 | grep -vE 'ExampleCommand|bash_completions.*Fprint');
+          fi
+version: 2
+jobs:
+  go-current:
+    docker:
+      - image: circleci/golang:1.8.3
+    <<: *base
+  go-previous:
+    docker:
+      - image: circleci/golang:1.7.6
+    <<: *base
+  go-latest:
+    docker:
+      - image: circleci/golang:latest
+    <<: *base